1. Field of the Invention
The present invention relates to a flow rate measuring device for outputting a signal in response to a flow rate, and more specifically to an air flow rate measuring device suitable for measuring, for example, the intake air amount in an internal combustion engine.
2. Description of the Related Art
FIG. 24 and FIG. 25 are a front elevational view and a longitudinal sectional view showing a conventional thermo-sensitive type flow rate sensor disclosed in Japanese Unexamined Patent Publication No. 8-313318, respectively.
In FIG. 24 and FIG. 25, a flow rate sensor 1 comprises a main passage 5 through which a fluid to be measured flows, a sensing passage 6 disposed in the main passage 5 substantially coaxially therewith, a flow rate sensing element 12A disposed in the sensing passage 6, a temperature compensating resistor 13 disposed in the vicinity of the inlet of the sensing passage 6 in the main passage 5 and an air flow regulating grid 7 disposed in the vicinity of the inlet in the main passage 5. Then, a circuit board 8 electrically connected to the flow rate sensing element 12A and the temperature compensating resistor 13 is accommodated in a circuit case 9. Further, a connector 10 for supplying power to the flow rate sensor 1 and taking out a flow rate sensing signal to the outside is disposed on the circuit case 9.
The flow rate sensing element 12A includes a ceramic substrate and a flow rate sensing resistor 11 which is composed of a comb-shaped platinum film formed on the ceramic substrate. Similarly, the temperature compensating resistor 13 is composed of a comb-shaped platinum film formed on the ceramic substrate.
In the conventional flow rate sensor 1 arranged as described above, a heating current flowing to the flow rate sensing resistor 11 of the flow rate sensing element 12A is controlled by a circuit formed in the circuit board 8 so that the average temperature of the flow rate sensing resistor 11 is made higher than the temperature of the fluid to be measured, which is sensed by the temperature compensating resistor 13, by a predetermined temperature. Then, the flow rate of the fluid to be measured can be sensed by measuring the heating current supplied to flow rate sensing resistor 11 and converting the heating current into a flow rate signal by making use of the cooling effect of the flow rate sensing resistor 11 cooled by the fluid to be measured and of the characteristics that the resistance value of the flow rate sensing resistor 11 is varied by temperature.
FIG. 26 shows the piping of an intake air system when the flow rate sensor 1 is generally used as an intake air flow rate sensor of an internal combustion engine for automobile. In many cases, the flow rate sensor 1 is disposed downstream of an air cleaner element 2 accommodated in an air cleaner case 3. The air cleaner element 2 is a filter composed of non-woven fabric, filter paper or the like and is disposed to capture dusts in air drawn by the internal combustion engine and to prevent them from entering the interior thereof. Dusts deposit on the air cleaner element 2 as an automobile travels and it is clogged therewith. Accordingly, differentia of the flow of the intake air having passed through the air cleaner element 2 is remarkable as compared with that of the flow thereof before the air cleaner element 2 is clogged, and thus the distribution of flow velocity of the intake air upstream of the flow rate sensor 1 greatly varies.
Since the flow rate sensing element 12A of the flow rate sensor 1 obtains flow information by sensing the flow velocity of air in a very small portion of the main passage 5, when the distribution of flow velocity of air upstream of the flow rate sensor 1 varies, an error arises in a flow rate sensing signal even if the flow velocity does not vary.
Incidentally, as the size of an engine room is reduced recently, there is an increasing need for the reduction in size of the flow rate sensor. To satisfy this need, proposed is a flow rate sensor of a so-called plug-in structure which does not include an air flow piping section (main passage 5) as disclosed in, for example, Japanese Unexamined Patent Publication No. 8-219838. However, since the flow rate sensor of the plug-in structure is not provided with the air flow piping section, it is difficult for the flow rate sensor to regulate an air flow using the air flow regulator (air flow regulating grid 7) as described above. Therefore, when the flow rate sensor of the plug-in structure is used as an intake air flow rate sensor of an internal combustion engine for automobile, an error is liable to arise in a flow rate sensing signal at the time the air cleaner element 2 is clogged.
Further, it is contemplated to mount an air flow regulator on an air cleaner case or an intake air pipe to improve the sensing accuracy of the flow rate sensor of the plug-in structure. In this case, the pitch of the air flow regulating elements of the air flow regulator must be made fine to obtain a sufficient air flow regulating effect. Making the pitch of the air flow regulating elements fine results in an increase of a pressure loss. Thus, there arises a problem that the amount of air which can be drawn by the internal combustion engine is reduced and the output thereof is lowered. Further, when the pitch of the air flow regulating elements is too fine, a problem arises in that they are clogged with small dusts having passed through an air cleaner element. In contrast, when the pitch of the air flow regulating elements is made coarse, not only an air flow regulating effect is lowered but also the thickness of a boundary layer and the frictional stress in a flow rate sensing unit are made uneven by eddies generated downstream and upstream of the air flow regulator. Accordingly, there arises a problem that a flow cannot be correctly measured because a flow rate sensing signal is disturbed.
Further, there is a problem that a manufacturing cost is increased because an air regulating structural member is provided in addition to the flow rate sensor.
A recent thermo-sensing type flow rate sensor employs a flow rate sensing element which is miniaturized to provide a prompt response. In this case, when an air flow regulator is disposed upstream of a flow rate sensing element, there is a problem that the flow rate sensing element is liable to be affected by the fluidic disturbance generated by the air flow regulator and a flow rate sensing accuracy is deteriorated by the further increase of a noise component included in a flow rate sensing signal.
In addition, the flow rate sensor of the plug-in structure is inserted into a hole opened to a main passage as shown in, for example, FIG. 1 and FIG. 2 of Japanese Unexamined Patent Publication No. 8-219838 and the sensing passage thereof is located approximately at the center of the cross section of the main passage. When the flow rate sensor is viewed from an upstream side, a support member is interposed between the sensing passage and the main passage to support the sensing passage. Accordingly, differentia of a flow resistance is caused in an up/down direction (the direction in which the support member extends from the inner wall surface of the main passage) about the sensing passage. Further, a flow rate sensor using a plate-shaped air flow regulating member disposed below a sensing passage is disclosed in Japanese Unexamined Patent Publication No. 10-332453. Differentia of a fluid resistance is caused in an upper and lower direction about the sensing passage also in this case. In the flow rate sensor arranged as described above, when a distribution of flow velocity varies upstream of the flow rate sensor, the flow velocity is made uneven in the up/down direction of the flow rate sensor and an error arises in a flow rate sensing signal.
A flow rate sensor arranged integrally with a fluid temperature sensing element using a thermistor or the like is disclosed in, for example, Japanese Unexamined Patent Publication No. 8-297040. When the flow rate sensor is viewed from an upstream side, the thermistor is mounted by being dislocated in a right or left side about a support member for supporting a sensing passage. In this case, since differentia of a fluid resistance is caused in the right to left direction about the support member, when a distribution of flow velocity varies upstream of the flow rate sensor, an error is liable to arise in a sensed flow.
The flow rate sensor, which uses the thermistor as the fluid temperature sensing element, is provided with a protector to prevent the damage of the thermistor when the flow rate sensor is inserted into an air flow piping section as disclosed in Japanese Unexamined Patent Publication No. 8-297040. However, there is a possibility that the thermistor is damaged when it is inserted into a hole formed at a sensing passage in the assembly of the flow rate sensor, and this is not taken into consideration.
Further, when a film-shaped flow rate sensing element is used as shown in Japanese Unexamined Patent Publication No. 10-142020, the flow rate sensing element is assembled so that it is substantially in parallel with the axial direction of a flow rate sensor (the axial direction of a sensing passage) as well as the surface thereof is substantially flush with a plate-shaped member extending into a sensing passage, and an end thereof is buried in a support member and fixed therein. Then, the flow rate sensing element is in electrical conduct to a control circuit section by a method of wire bonding or the like. Further, the flow rate sensing element is disposed substantially at the center of the cross section of the sensing passage where a distribution of flow velocity is made uniform. When it is intended to perform a response from the flow rate sensor at a high speed, the miniaturization of the flow rate sensing element is effective for the purpose. However, the flow rate sensing element is miniaturized, a protective member for protecting the electric junction at which the flow rate sensing element is electrically connected to the control circuit section must be exposed to the sensing passage to dispose the flow rate sensing element at the center of the sensing passage. In this case, when the sensing passage is viewed from an upstream side, differetia of a flow resistance, which is uneven in an up/down direction, is caused by the protective member. Therefore, when the distribution of flow velocity varies upstream of the flow rate sensor, an error arises in a sensed flow.
An object of the present invention, which was made to solve the above problems, is to obtain a flow rate sensor which can correctly sense a flow even if a distribution of flow velocity varies upstream of the flow rate sensor, has a small pressure loss and is less expensive including a manufacturing cost.
In order to achieve the above object, according to one aspect of the present invention, there is provided a flow rate sensor comprising a flow rate sensing element for sensing the flow rate of a fluid, a sensing passage into which the fluid is introduced and in which the flow rate sensing element is disposed, a support member for supporting the sensing passage and a circuit case in which an electronic circuit unit for controlling the flow rate sensing element is accommodated, wherein the sensing passage, the support member and the circuit case are formed integrally with each other and the support member extends into a main passage, through which the fluid flows, through a hole opened to the main passage so as to position the sensing passage in the main passage. The flow rate sensor further comprises a structural member whose the outside shape is formed to have a fluid resistance approximately similar to that of the portion of the support member extending from the hole, wherein the structural member is disposed at a position approximately symmetrical with the portion of the support member extending from the hole about the sensing passage.
According to another aspect of the present invention, there is provided a flow rate sensor comprising a flow rate sensing element for sensing the flow rate of a fluid, a sensing passage into which the fluid is introduced and in which the flow rate sensing element is disposed, a support member for supporting the sensing passage, a circuit case in which an electronic circuit unit for controlling the flow rate sensing element is accommodated and a fluid temperature sensing element for sensing the temperature of the fluid, wherein the sensing passage, the support member and the circuit case is formed integrally with each other and the support member extends into a main passage, through which the fluid flows, through a hole opened to the main passage so as to position the sensing passage and the fluid temperature sensing element in the main passage. The flow rate sensor further comprises a protector for protecting the fluid temperature sensing element, and a structural member whose the outside shape is formed to have a fluid resistance approximately similar to that of the protector, wherein the protector and the structural member are disposed at positions approximately symmetrical with respect to the support member for supporting the sensing passage.
According to still another aspect of the present invention, there is provided a flow rate sensor comprising a flow rate sensing element for sensing the flow rate of a fluid, a sensing passage into which the fluid is introduced and in which the flow rate sensing element is disposed, a support member for supporting the sensing passage, a sensing assistant member disposed so as to extend into the sensing passage for supporting the flow rate sensing element and a circuit case in which an electronic circuit unit for controlling the flow rate sensing element is accommodated. The flow rate sensor further comprises a protective member disposed so as to project into the sensing passage for protecting an electric junction, at which the electronic circuit unit is connected to the flow rate sensing element, from the fluid in cooperation with the sensing assistant member, and a structural member the outside shape of which is formed to have a fluid resistance approximately similar to that of the portion of the protective member projecting into the sensing passage, the structural member being located at a position approximately symmetrical with the portion of the protective member projecting into the sensing passage about the axial center of the sensing passage.